restic: generic implemention of AsBackend

internal/migrations/s3_layout.go

@@ -21,26 +21,9 @@ func init() {
 // "default" layout.
 type S3Layout struct{}
 
-func toS3Backend(b restic.Backend) *s3.Backend {
-	for b != nil {
-		if be, ok := b.(*s3.Backend); ok {
-			return be
-		}
-
-		if be, ok := b.(restic.BackendUnwrapper); ok {
-			b = be.Unwrap()
-		} else {
-			// not the backend we're looking for
-			break
-		}
-	}
-	debug.Log("backend is not s3")
-	return nil
-}
-
 // Check tests whether the migration can be applied.
 func (m *S3Layout) Check(_ context.Context, repo restic.Repository) (bool, string, error) {
-	be := toS3Backend(repo.Backend())
+	be := restic.AsBackend[*s3.Backend](repo.Backend())
 	if be == nil {
 		debug.Log("backend is not s3")
 		return false, "backend is not s3", nil
@@ -92,7 +75,7 @@ func (m *S3Layout) moveFiles(ctx context.Context, be *s3.Backend, l layout.Layou
 
 // Apply runs the migration.
 func (m *S3Layout) Apply(ctx context.Context, repo restic.Repository) error {
-	be := toS3Backend(repo.Backend())
+	be := restic.AsBackend[*s3.Backend](repo.Backend())
 	if be == nil {
 		debug.Log("backend is not s3")
 		return errors.New("backend is not s3")

internal/migrations/s3_layout_test.go

@@ -1,27 +0,0 @@
-package migrations
-
-import (
-	"testing"
-
-	"github.com/restic/restic/internal/backend/mock"
-	"github.com/restic/restic/internal/backend/s3"
-	"github.com/restic/restic/internal/cache"
-	"github.com/restic/restic/internal/test"
-)
-
-func TestS3UnwrapBackend(t *testing.T) {
-	// toS3Backend(b restic.Backend) *s3.Backend
-
-	m := mock.NewBackend()
-	test.Assert(t, toS3Backend(m) == nil, "mock backend is not an s3 backend")
-
-	// uninitialized fake backend for testing
-	s3 := &s3.Backend{}
-	test.Assert(t, toS3Backend(s3) == s3, "s3 was not returned")
-
-	c := &cache.Backend{Backend: s3}
-	test.Assert(t, toS3Backend(c) == s3, "failed to unwrap s3 backend")
-
-	c.Backend = m
-	test.Assert(t, toS3Backend(c) == nil, "a wrapped mock backend is not an s3 backend")
-}

internal/restic/backend.go

@@ -75,6 +75,23 @@ type BackendUnwrapper interface {
 	Unwrap() Backend
 }
 
+func AsBackend[B Backend](b Backend) B {
+	for b != nil {
+		if be, ok := b.(B); ok {
+			return be
+		}
+
+		if be, ok := b.(BackendUnwrapper); ok {
+			b = be.Unwrap()
+		} else {
+			// not the backend we're looking for
+			break
+		}
+	}
+	var be B
+	return be
+}
+
 // FileInfo is contains information about a file in the backend.
 type FileInfo struct {
 	Size int64

internal/restic/backend_test.go

@@ -0,0 +1,38 @@
+package restic_test
+
+import (
+	"testing"
+
+	"github.com/restic/restic/internal/restic"
+	"github.com/restic/restic/internal/test"
+)
+
+type testBackend struct {
+	restic.Backend
+}
+
+func (t *testBackend) Unwrap() restic.Backend {
+	return nil
+}
+
+type otherTestBackend struct {
+	restic.Backend
+}
+
+func (t *otherTestBackend) Unwrap() restic.Backend {
+	return t.Backend
+}
+
+func TestAsBackend(t *testing.T) {
+	other := otherTestBackend{}
+	test.Assert(t, restic.AsBackend[*testBackend](other) == nil, "otherTestBackend is not a testBackend backend")
+
+	testBe := &testBackend{}
+	test.Assert(t, restic.AsBackend[*testBackend](testBe) == testBe, "testBackend was not returned")
+
+	wrapper := &otherTestBackend{Backend: testBe}
+	test.Assert(t, restic.AsBackend[*testBackend](wrapper) == testBe, "failed to unwrap testBackend backend")
+
+	wrapper.Backend = other
+	test.Assert(t, restic.AsBackend[*testBackend](wrapper) == nil, "a wrapped otherTestBackend is not a testBackend")
+}